Systems and methods for measuring magnetostriction in magnetoresistive elements

ABSTRACT

A system for directly measuring a magnetostriction value of a magnetoresistive element includes a fixture for receiving a substrate carrying one or more magnetoresistive elements. A magnet assembly applies a first magnetic field parallel to the substrate, and a magnetic alternating field perpendicular to the substrate and parallel to magnetoresistive layers of the elements. A stress-inducing mechanism applies a mechanical stress to the substrate, the stress being oriented parallel to the substrate. A measuring subsystem measures a signal from at least one of the magnetoresistive elements.

FIELD OF THE INVENTION

The present invention relates in general to the measurement of themagnetostriction constant. More specifically, the invention relates tosuch measurements in magnetoresistive devices.

BACKGROUND OF THE INVENTION

There are many situations in which there is a need to measure a magneticfield. Among such situations are the measurement of position orproximity of a magnetized portion of a structure, the reactant of storedmagnetic information, the measurement of current flows without the needof a measuring device in the current flow path, etc.

Many of the magnetic effects in such situations are relatively small andtherefore require a sensitive magnetic sensor. A magnetic sensor capableof sensing such small magnetic field perturbations, and which iseconomical to fabricate, is provided on the basis of themagnetoresistive effect. Such magnetoresistive material based magneticsensors can be fabricated as thin films when using monolithic integratedcircuit fabrication techniques, and so can not only be made economicallybut also made quite small in size. A magnetoresistive material basedmagnetic sensor is arranged by providing a magnetoresistive material tobe used as an electrical resistor. A current is passed therethrough, andthe voltage there across will depend on the effective resistance of thematerial over the path in which the current flows. That resistance valuewill depend in turn on the state of the magnetization of the material.If the magnetization is parallel to the current flow, what is the casefor Anisotropic Magnetoresistance (AMR), the material will exhibit amaximum resistance, and it will exhibit a minimum resistance formagnetization perpendicular to the current flow.

For Giant Magnetoresistance (GMR), the maximum resistance is forparallel alignment of the magnetization of adjacent magnetic layers,separated by non-magnetic interface layers. A spin valve system consistsof two magnetic layers, a free layer and a pinned layer. The pinning canbe made by an antiferromagnetic layer or by antiferromagneticallycoupled pinning layers.

The current in such systems can be current-in-plane (CIP) orcurrent-perpendicular-to-plane (CPP). The CPP structure is normally usedin tunneling devices (Tunneling Magnetoresistance—TMR), where thenon-magnetic interface layer consists of an electrically resistiveisolator material.

In the magnetoresistive device there is typically a free rotating layerwith an effective magnetization. An external field acting on themagnetoresistive material will rotate the magnetization directiontherein to change the resistance of the layer system as a result. Thechanged resistance of the material carrying the current causes a voltagedrop change across the resistor which can be sensed as an indication ofthe magnitude of the external field.

The effective resistance of such a film will vary as the square of thecosine of the angle between the effective magnetization direction andthe current flow direction through the material in the AMR case and asthe cosine of the angle of adjacent layers in the GMR or TMR case. Thetotal resistance, however, is usually not of interest but rather thechange in resistance in response to a change in the applied externalmagnetic field. In the AMR case, this change is often best measured at apoint along the squared cosine response curve where the curveapproximates a linear function.

To provide operation on such a linear portion of the response curverequires that there be an initial angle between the direction of currentflow and the nominal direction of magnetization in the absence of anyexternally applied fields. This can be accomplished in alternative waysin a bias arrangement. The magnetoresistive material can be placed onthe device substrate as a continuous resistor in a “herringbone” patternor other design of continuously connected multiple inclines, with theangle of incline being approximately 45° with respect to the directionof extension of the resistor. There then must be provided a source for amagnetic bias field which is oriented in a direction which is 90° to thedirection of the extension of the resistor.

Another method is to provide a linear strip of magnetoresistivematerial, with additional individual conductors across that strip at anangle of 45° with respect to the direction of the strip. This, ineffect, causes the current to flow at an angle through themagnetoresistive strip with respect to the direction of elongation ofthe strip itself. This latter configuration is often called a “barberpole” sensor because of its configuration, and such an arrangement caneliminate the need for an external source of a magnetic bias field.

In magnetic recording heads the magnetization of the sensing layer of anAMR sensor is rotated by 45° in relation to the sense current by thestray field of an adjacent magnetic layer magnetized perpendicular tothe direction of the sensor strip. This layer can be a hard magneticmaterial (hard bias layer) or a soft magnetic material (soft adjacentlayer) magnetized by the sense current.

In GMR or TMR elements the magnetization of the free layer has to bedirected parallel to the strip direction. This is normally done by ahard bias layer placed on each side of the sensor. The magnetization ofthe pinned layers will be fixed perpendicular to the strip direction byantiferromagnetic coupling.

Magnetostriction is an essential parameter for controlling the magneticproperties of thin films and multilayers. Magnetostriction describes thechange in length of a magnetic material by magnetic reversal.

In magnetic recording elements it is important to have homogeneouslymagnetized magnetic layers, especially the sensing layer (free layer) inthe sensing layer stack. Inhomogeneously magnetized regions, likevortices or magnetic domains, cause instabilities in the recordingsignal. Therefore, the magnetic layers are aligned by local magneticfields (exchange coupling field, hard bias field). Local inhomogeneitiescan be caused by magnetostrictive anisotropy. Therefore, themagnetostriction has to be controlled very precisely.

Various experimental methods have been developed for investigating themagnetoelastic properties of thin films. One of them is the directmeasurement by the so-called “cantilever method”. A change inmagnetization leads to a change in length which with thin films causesbending of the substrate. This is, e.g., described in E. du Trémolet deLacheisserie et al., “Magnetostriction and internal stresses in thinfilms: the cantilever method revisited”, Journal of Magnetism andMagnetic Materials 136 (1994), pp. 189-196.

Another possibility is the indirect measurement by means of the straingauge method, which creates mechanical stresses in a magnetic film. Themagnetic anisotropy changes through magnetostrictive coupling. This is,e.g., described in D. Markham et al., “Magnetostrictive measurement ofmagnetostriction in Permalloy”, IEEE Transactions on Magnetics, vol. 25,no. 5, September 1989, pp. 4198-4200.

An apparatus for measuring the magnetostriction constant of a magneticmembrane is disclosed in Patent Abstracts of Japan, JP 62106382 A2.

Kenji Narita et al., IEEE Transactions on Magnetics, vol. Mag-16, no. 2,March 1980, pp. 435-439, disclose a method to measure the saturationmagnetostriction of a thin amorphous ribbon by means of Small-AngleMagnetization Rotation (SAMR).

However, no method is known to measure the magnetization changes usingthe magnetoresistive effect of magnetic sensors directly, so that thereal environment of the sensor is reflected. Therefore, there is still aneed for improvement of such methods.

SUMMARY OF THE INVENTION

A system for directly measuring a magnetostriction value of amagnetoresistive element, such as a magnetostriction constant, includesa fixture for receiving a substrate carrying one or moremagnetoresistive elements. A magnet assembly applies a first magneticfield, preferably a DC field, parallel to the substrate. The magnetassembly also applies a magnetic alternating field perpendicular to thesubstrate and parallel to magnetoresistive layers of the elements. Amechanism for applying a mechanical stress to the substrate creates astress oriented parallel to the substrate. A measuring subsystem, whichmay be locked to the frequency of the alternating field, measures asignal from at least one of the magnetoresistive elements.

The system may further include a controller for changing the firstmagnetic field, and a computing device for calculating amagnetostriction constant of the at least one magnetoresistive elementbased in part on e.g., a change of mechanical stress anisotropy due toapplication of the mechanical stress and the change in the firstmagnetic field.

The substrate can be a row or a wafer, which may in turn carry aplurality of the magnetoresistive elements such as AnisotropicMagnetoresistance (AMR)-, Giant Magnetoresistance (GMR)- or TunnelingMagnetoresistance (TMR)-based sensors, and may further include shieldinglayers, etc. The magnetoresistive elements may also be magnetic memoryelements.

The signal from the at least one of the magnetoresistive elements ismeasured before the mechanical stress is applied. After applying themechanical stress, the first magnetic field is changed until the signalbeing measured co-currently thereto about matches the signal measuredbefore applying the mechanical stress.

The mechanism for applying the mechanical stress may cause the substrateto bend. An illustrative mechanism is an electronically or manuallycontrolled micrometer screw. Alternatively, the mechanism for applyingthe mechanical stress may be a heat source, a heat sink, or a piezolayer.

Methods for directly measuring a magnetostriction value of amagnetoresistive element are also disclosed.

Other aspects of the present invention will become apparent from thefollowing detailed description, which, when taken in conjunction withthe drawings, illustrate by way of example the principles of theinvention

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts a system for measuring the magnetostrictionconstant according to a preferred method of the present invention.

FIG. 2 is a flow diagram of a process for measuring the magnetostrictionconstant according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the present invention, the magnetostriction constant (MS) inAnisotropic Magnetoresistance (AMR), Giant Magnetoresistance (GMR)and/or Tunneling Magnetoresistance (TMR) (in general XMR) basedelements, like magnetic recording heads, magnetic field sensors and thelike, is measured by small angle magnetization rotation (SAMR). Theelectrical signal of the sensor is used to measure the magnetizationrotation caused by an external field. In Magnetoresistance (MR) devices,the magnetization is biased by various methods, e.g., hard bias,antiferromagnetic exchange coupling, barber pole, etc. For the proposedMs measurement the bias fields (hard bias, soft bias, exchange field)can be supported by an additional external DC field (HDC) which isaligned parallel to the applied stress. If the stress in the thin filmis changed, the sensor signal will also change due to magnetostrictivecoupling. However, the change of the sensor signal can be compensated bychanging the external DC field. For shielded elements the external fieldis calibrated in order to reflect the influence of demagnetizing effectsfrom the shielding layers. The stress can be applied on wafer or rowlevel by bending or by any other means like, e.g., temperature changeinduced by heat source or heat sink, piezo layer, etc.

The methodology according to the invention is not only applicable tomagnetic recording heads but can also be used with other magneticdevices such as magnetic field sensors and magnetic random accessmemories (MRAMs). However, for the sake of simplicity, it is explainedin the following description with respect to magnetic recording heads.

FIG. 1 schematically depicts a system for measuring the magnetostrictionconstant according to the methodology of the present invention. Asshown, a row or a wafer 10 is inserted into a bending fixture, e.g., adeflection gauge 12, the row or wafer carrying XMR elements formedthereon. Next, a first magnetic field, preferably a DC-field, is appliedparallel to the row or wafer 10, i.e., in the direction of the x-axisshown in FIG. 1. A magnetic alternating field is applied perpendicularto the row or wafer 10 and parallel to the magnetoresistive layers,i.e., in the direction of the y-axis shown in the Figure. Thisalternating field is preferably sinusoidal having the frequency f. Ameasuring subsystem 14 measures a signal at the magnetoresistiveelement, e.g., XMR element, this signal being proportional to theamplitude of the alternating field having the frequency f. To do this ina simple way, a lock-in amplifier 14 can be used which is locked to thefrequency of the alternating field. A mechanical stress is created inthe layers of the XMR element parallel to the x-direction by bending therow or wafer 10, e.g., by means of a micrometer screw 16, pneumaticallyor hydraulically actuated piston, solenoid, etc. The screw or otherstress-inducing mechanism can be controlled electronically via line 20by a control unit 48. Due to the magnetoelastic interaction in thesensor layer of the XMR element, the magnetic anisotropy will change.This, in turn, will lead to a change in the amplitude of the signal thatis measured at the lock-in amplifier 14. Finally, the applied magneticDC-field in the direction of the x-axis is changed by a suitable controlcircuit until the measuring signal at the lock-in amplifier againreaches the value that has been measured without having appliedmechanical stress. The magnet assembly 22 above the row/wafer deflectionfixture 12 is powered by an AC power supply 42 for magnetic fieldgeneration in the y-direction, and a DC power supply 24 for generatingthe DC compensation field in the x-direction via lines 28 and 30. TheXMR− element is powered via line 32 by a constant current source 34. Thesense output, the voltage across the XMR− resistor, is fed via line 36into the lock-in amplifier 14, being locked to the excitation frequencyof the magnetic AC field via line 38, as already mentioned above. Thewhole measurement assembly can be controlled by a computer 40 via bus26. Note also that the computer 40 itself can perform some or all of thefunctions of, and/or replace, the various components 34, 14, 42, 24, 48in hardware, software, or combinations thereof.

FIG. 2 graphically illustrates a method 200 for directly measuring themagnetostriction constant of a magnetoresistive element according topreferred embodiment. In operation 202, a substrate carrying one or moremagnetoresistive elements is provided. The substrate is inserted into oronto a bending fixture in operation 204. In operation 206, a magnetic DCfield is applied parallel to the substrate, while in operation 208, amagnetic alternating field is applied perpendicular to the substrate andparallel to the magnetoresistive layers of the elements. A signal fromthe element is measured in operation 210. A mechanical stress orientedparallel to the substrate is applied in operation 212 by bending thesubstrate. In operation 214, the magnetic DC field is changed until thecurrent signal reading about matches the signal measured before applyingthe mechanical stress. In operation 216, the magnetostriction constantλ_(s) (or other value) is calculated. Illustrative formulae forcalculating the magnetostriction constant and other values are presentedbelow.

The magnetostriction λ_(s) is defined by the following formula

$\begin{matrix}{{\frac{3}{2}\lambda_{s}*{\Delta\sigma}} = {\frac{1}{2}\Delta\; H_{k,\sigma}M_{s}}} & (I)\end{matrix}$that indicates that the magnetoelastic energy density (left side of theequation) is identical to the magnetic anisotropic energy density (rightside of the equation).

The change of mechanical stress anisotropy Δσ is connected with thestrain change

${\Delta ɛ} = \frac{\Delta\; l}{l}$(the relative elongation caused by bending of the row or wafer), byHooke's law (being restricted to homogeneous and isotropic materials).

${{\Delta\sigma}_{x} = {\frac{E}{1 - v^{2}}( {{\Delta ɛ}_{x} - {v\;{\Delta ɛ}_{y}}} )}};{{\Delta\sigma}_{y} = {\frac{E}{1 - v^{2}}( {{\Delta ɛ}_{y} - {v\;{\Delta ɛ}_{x}}} )}}$

The voltage change Δσ is calculated from the mechanical parameters ofthe deflection:

$\begin{matrix}{{\Delta\sigma} = {{{\Delta\sigma}_{x} - {\Delta\sigma}_{y}} = {\frac{E}{1 - \upsilon}( {{\Delta ɛ}_{x} - {\Delta ɛ}_{y}} )}}} & ({II})\end{matrix}$

The following methods can be used to obtain special mechanicalparameters:

-   1) The strain can be calculated from the deflection (b in FIG. 1) by    the following expression:

$\begin{matrix}{{{\Delta ɛ}_{x} = {\frac{3d_{s}*b}{2L^{2}}\lbrack {1 - \frac{x}{L}} \rbrack}},} & ({III})\end{matrix}$

-   -   where L is the bending length (cf. FIG. 1), d_(s) is the        substrate thickness (cf. FIG. 1), x=0 at the center of the        strain gauge, and Δε_(y)=0.

-   2) Measuring the surface curvature by scanning a laser beam over the    sample surface. The laser is reflected from the row or wafer    surfaces to a position sensitive optical device.

The strain Δε is determined from the deflection b or the surfacecurvature in (II).

The measurement of the field of anisotropy follows from the totalenergy:

$\begin{matrix}{E = {{H_{x}M_{s}\cos\;\vartheta} - {H_{y}M_{s}\sin\;\vartheta} + {\frac{1}{2}H_{k}M_{s}\sin^{2}\vartheta} + {\frac{1}{2}\langle N_{demag} \rangle M_{s}^{2}\sin^{2}\vartheta}}} & ({IV})\end{matrix}$

This term includes the energy in the external fields (H_(x), H_(y)), theuniaxial anisotropy (H_(k)), which is composed of the induced anisotropyand the magnetostrictive anisotropy, as well as the form anisotropy,which takes into account the distribution of the magnetization of thelayer to be measured.

From the condition of equilibrium dE/d=0 follows:

$\begin{matrix}{{\sin\;\vartheta} = {\frac{H_{y}}{H_{x} + H_{k} + {\langle N_{demag} \rangle M_{s}}}.}} & (V)\end{matrix}$

Given a periodic excitation field H_(y)=H_(yo) sin (Φt), themagnetization will fluctuate around a state of equilibrium. Theresistance of a magnetoresistive element changes with the identicalfrequency.

A mechanical tension changes the anisotropic field H_(k). This, in turn,causes a change in resistance and a change in amplitude of thefluctuations of the magnetization. These changes are compensated bymeans of the external field H_(x) and the original state of equilibriumis restored. The following equation applies:ΔH _(k,σ) =ΔH _(x)  (VI).

The calculation of λ_(s) is straight forward. The strain Δε can becalculated, e.g., from (III). The stress anisotropy Δσ is then derivedfrom (II). From (I) the magnetostriction constant can be calculated byinserting Δσ, and ΔH_(k,σ) from (IV).

The saturation magnetization M_(s) and the elasticity constants E and νare inserted into equation (I) as constants.

One skilled in the art will appreciate that calculations using theforegoing equations can be performed in hardware or software, such as bythe computer 40 in FIG. 1. The calculation details and results can thenbe stored or transmitted in electronic form, tabulated for multipleelements on the substrate and/or for elements across various substrates,displayed in visual form, output on paper, etc.

Also, as an alternative, the first magnetic field can be a static fieldfrom a hard magnet. To vary the field, the distance between the magnetand the substrate can be varied.

The present invention suggests using the MR-effect of the magneticsensors directly for the measurement of the magnetization changes. Themethodology according to the present invention has the followingadvantages, among others:

-   -   The use of a compensation method guarantees a fixed        magnetization state, which avoids errors by local magnetization        distributions in structured films. Bias fields, e.g., softbias        in anisotropic magnetoresistive sensor (AMR), longitudinal        magnetically hard fields (hardbias) or exchange fields adjacent        magnetic layers do not have any effect.    -   The measurement is simple and fast.    -   The MR effect is very sensitive.    -   The measurement can be performed in the sensor elements        themselves. There is no need to have separately-manufactured        monitor layers.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. Thus, the breadth and scope of a preferred embodiment shouldnot be limited by any of the above-described exemplary embodiments, butshould be defined only in accordance with the following claims and theirequivalents.

1. A system for directly measuring a magnetostriction value of amagnetoresistive element, the system comprising: a fixture for receivinga substrate carrying one or more magnetoresistive elements; a magnetassembly for applying a first magnetic field parallel to the substrate,and for applying a magnetic alternating field perpendicular to thesubstrate and parallel to magnetoresistive layers of the elements; amechanism for applying a mechanical stress to the substrate, the stressbeing oriented parallel to the substrate; and a measuring subsystem formeasuring a signal from at least one of the magnetoresistive elements.2. A system according to claim 1, wherein the substrate is a row or awafer.
 3. A system according to claim 2, wherein the row or wafercarries a plurality of the magnetoresistive elements.
 4. A systemaccording to claim 1, wherein the first magnetic field is a DC field. 5.A system according to claim 1, wherein the measuring subsystem is lockedto a frequency of the alternating field.
 6. A system according to claim1, wherein the signal from the at least one of the magnetoresistiveelements is measured before the mechanical stress is applied; wherein,after applying the mechanical stress, the first magnetic field ischanged until the signal being measured co-currently thereto aboutmatches the signal measured before applying the mechanical stress.
 7. Asystem according to claim 1, wherein the mechanism for applying themechanical stress causes the substrate to bend.
 8. A system according toclaim 7, wherein the mechanism for applying the mechanical stress is amicrometer screw.
 9. A system according to claim 8, wherein themicrometer screw is electronically controlled.
 10. A system according toclaim 1, wherein the mechanism for applying the mechanical stress is aheat source.
 11. A system according to claim 1, wherein the mechanismfor applying the mechanical stress is a piezo layer.
 12. A systemaccording to claim 1, further comprising a controller for changing thefirst magnetic field.
 13. A system according to claim 12, furthercomprising a computing device for calculating a magnetostrictionconstant of the at least one magnetoresistive element based in part on achange of mechanical stress anisotropy due to application of themechanical stress and the change in the first magnetic field.
 14. Asystem according to claim 1, wherein the at least one magnetoresistiveelement includes shielding layers, wherein the first magnetic field iscalibrated to reflect an influence of a demagnetizing effect of theshielding layers on the element.
 15. A system according to claim 1,wherein the magnetoresistive element is an Anisotropic Magnetoresistance(AMR)-, Giant Magnetoresistance (GMR)- or Tunneling Magnetoresistance(TMR)-based sensor.
 16. A system according to claim 1, wherein themagnetoresistive elements are magnetic memory elements.
 17. A system fordirectly measuring a magnetostriction value of a magnetoresistiveelement, the system comprising: a bending fixture for receiving asubstrate carrying one or more magnetoresistive elements; a magnetassembly for applying a magnetic direct current (DC) field parallel tothe substrate, and for applying a magnetic alternating fieldperpendicular to the substrate and parallel to magnetoresistive layersof the elements; a mechanism for applying a mechanical stress to thesubstrate by bending the substrate, the stress being oriented parallelto the substrate; a control circuit for changing the DC magnetic field;and a measuring subsystem for measuring a signal from at least one ofthe magnetoresistive elements prior to application of the mechanicalstress, after application of the mechanical stress, and during a timeperiod when the DC magnetic field is changed.
 18. A system for directlymeasuring a magnetostriction value of a magnetoresistive element, thesystem comprising: a bending fixture for receiving a substrate carryingone or more magnetoresistive elements; a magnet assembly for applying amagnetic direct current (DC) field parallel to the substrate, and forapplying a magnetic alternating field perpendicular to the substrate andparallel to magnetoresistive layers of the elements; a DC power supplyfor providing power to the magnet assembly; an alternating current (AC)power supply for providing power to the magnet assembly; a mechanism forapplying a mechanical stress to the substrate by bending the substrate,the stress being oriented parallel to the substrate; a measuringsubsystem for measuring a signal from at least one of themagnetoresistive elements prior to application of the mechanical stress,after application of the mechanical stress, and during a time periodwhen the DC magnetic field is changed; a control circuit for changingthe DC magnetic field until the signal currently being measured by themeasuring subsystem about matches a signal measured before applying themechanical stress; and a computing device for calculating amagnetostriction constant of the at least one magnetoresistive elementbased in part on a change of mechanical stress anisotropy due toapplication of the mechanical stress and the change in the DC magneticfield.
 19. A method for directly measuring a magnetostriction value of amagnetoresistive element, the method comprising: providing a substratecarrying one or more magnetoresistive elements; placing the substrate ona fixture; applying a first magnetic field parallel to the substrate;applying a magnetic alternating field perpendicular to the substrate andparallel to magnetoresistive layers of the elements; measuring a signalfrom the element; applying a mechanical stress to the substrate, thestress being oriented parallel to the substrate; and changing the firstmagnetic field until the signal currently being measured about matches asignal measured before applying the mechanical stress.
 20. A methodaccording to claim 19, wherein the substrate is a row or a wafer.
 21. Amethod according to claim 20, wherein the row or wafer carries aplurality of the magnetoresistive elements.
 22. A method according toclaim 19, wherein the mechanical stress causes the substrate to bend.23. A method according to claim 22, wherein the mechanical stress isapplied by a micrometer screw.
 24. A method according to claim 19,wherein the magnetoresistive element is an Anisotropic Magnetoresistance(AMR)-, Giant Magnetoresistance (GMR)- or Tunneling Magnetoresistance(TMR)-based sensor.
 25. A method for directly measuring amagnetostriction value of a magnetoresistive element, the methodcomprising: providing a substrate carrying one or more magnetoresistiveelements; placing the substrate on a bending fixture; applying amagnetic DC field parallel to the substrate; applying a magneticalternating field perpendicular to the substrate and parallel tomagnetoresistive layers of the elements; measuring a signal from atleast one element; applying a mechanical stress to the substrate bybending the substrate, the stress being oriented parallel to thesubstrate; changing the magnetic DC field until the signal currentlybeing measured about matches a signal measured before applying themechanical stress; and calculating a magnetostriction value of theelement.